In the field of indoor wireless networks, visible-light communications (VLC) is garnering increasing attention. One of the type of emitters used in this technology are light-emitting diodes, which can synergistically provide both illumination and data transmission.
One possible transmission mode for VLC is known as colour-shift keying (CSK). CSK supports visible-light communications using multi-colour light sources and photo detectors. The data to be transmitted is thereby encoded with mixed colours.
The IEEE 802.15.7 task group is currently working towards a global VLC standard, in which CSK coding is explicitly covered. According to a draft standard IEEE 802.15.7, a CSK signal is generated by using three colour light sources.
A CSK constellation is defined by a combination of three colour bands of the respective three colour light sources. The CSK constellation is operated in a triangle-shaped constellation diagram in x-y colour coordinates. The triangular shape stems from the fact that three emitters of different colour are use. Three vertices of the CSK constellation diagram are decided by the centre wavelength of the three colour bands in which the three emitters reside. How cases, in which the emitters lie in multiple bands, are decided is described in the current version of the IEEE 802.15.7 draft.
Within the CSK constellation, constellation points are defined. Constellation points, hereinafter also referred to as symbol points, latter symbol points being synonymous with constellation points, are defined by a design rule within draft standard IEEE 802.15.7.
By a data-mapping rule, which is also provided within draft standard IEEE 802.15.7, a unique binary symbol value is assigned to each constellation point in order to provide a coding scheme, by which a particular >>colour<<, i.e. a particular combination of the three colours, is assigned to a unique symbol value.
Applying this coding scheme, an optical transmitter is enabled to encode an optical signal by a stream of data, the optical signal being a combination of the three colours. Conversely, an optical receiver is enabled to decode the stream of data by a received optical signal by applying said coding scheme.
Although an assignment of a specific binary symbol value to a specific constellation point may be employed arbitrarily in a first approach, some assignments may exhibit advantages compared to other assignments.
If optical signals received by the receiver are obscured by noise, a wrong symbol might be decoded. In order to minimise the entailed bit errors one needs to minimise a number of flipped bits between symbols assigned to neighbouring constellation points.
Although the publication Yokoi, A: >>CSK Gray coding<<, IEEE 802, document #15-11-0010-01-007, 2011, has suggested an improved assignment of binary symbol values leading to a reduced number of flipped bits between symbols assigned to neighbouring constellation points, those suggested assignments are not yet optimal regarding the need in the art to achieve a minimum of said number of flipped bits between symbols assigned to neighbouring constellation points.